1. Field of the Invention
The present invention relates to a semiconductor device having a circuit comprising a thin film transistor (abbreviated as TFT hereinafter) formed by using a semiconductor thin film. In particular, it relates to a semiconductor device using a reverse stagger TFT.
The semiconductor device used herein means any device that can function utilizing semiconductor characteristics, and involves not only a single element, such as a TFT, but also a semiconductor circuit and an electro-optical apparatus, as well as an electronic apparatus carrying them as a part.
2. Description of Related Art
In recent years, a semiconductor device is receiving attention, in which a TFT is formed on a substrate by using a semiconductor thin film having crystallinity, and a circuit is formed with the TFT. As the semiconductor thin film, polycrystalline silicon (sometimes called polysilicon) is frequently used, and on the other hand, a compound semiconductor represented by SixGe1xe2x88x92x (0 less than x less than 1) is also studied.
While a TFT using a polysilicon film has been developed and is being subjected practical application, there is room for improvement in film quality and mass productivity, and further technical development is required. Under the circumstances, the inventors have proposed a technique described in JP-A-7-130652 and U.S. Pat. No. 5,643,826 (which is a counterpart of JP-A-7-130652), as a means for simultaneously solving the improvement in film quality of polysilicon and the improvement in mass productivity.
In the technique described in JP-A-7-130652, a catalyst element accelerating crystallization of silicon is added to an amorphous semiconductor film (a representative example thereof is amorphous silicon), and the crystallization is carried out by utilizing the function of the catalyst element. As a result, the temperature and time required for the crystallization be decreased, and the throughput is considerably improved. Furthermore, it has been confirmed that the resulting polysilicon has a remarkably high crystallinity, and the electric characteristics of the TFT are also considerably improved.
However, since nickel (Ni), which is the most effective as the 10 catalyst element, is a metallic element, there is a possibility in that nickel adversely affects the TFT characteristics when it remains in the polysilicon. Accordingly, the inventors have considered that removal of nickel is necessary after completion of the crystallization, and have developed a technique conducting gettering of the catalyst element. Such a method is described in JP-A-9-312260 or a copending application Ser. No. 08/785,489 filed on Jan. 17, 1997, which is a counterpart application of JP-A-9-312260.
The gist of the techniques described in these publications resides in the use of a metallic element, such as nickel, as a catalyst element for accelerating the crystallization, and thus the catalyst element is not necessarily present after the formation of polysilicon.
The invention has been developed in view of the problems described above, and an object of the invention is to provide a technique for forming a semiconductor film having high crystallinity by a production method excellent in mass productivity.
Another object of the invention is to improve an yield and a production cost of the semiconductor device by constituting a circuit with a TFT using the semiconductor film.
In the invention, germanium (Ge), which is a semiconductor, is used as a catalyst element accelerating the crystallization of silicon, so as to provide a process in that gettering is not necessary. Because germanium has the properties extremely close to silicon, it is present in silicon in conditions of good compatibility. That is, germanium has an advantage in that it does not adversely affect the TFT characteristics even if it is not removed after used as a catalyst element.
The invention is based on the technique in that germanium is added to an amorphous silicon film, and the amorphous silicon is crystallized by utilizing the catalytic action of germanium. According to the technique, the crystallization at a low temperature, decrease of the processing time, and shortening of the process are simultaneously realized.
Because germanium is present in silicon in conditions of good compatibility, the crystallinity is extremely high in comparison to the cases using other catalyst elements. Germanium continuously changes the band gap of silicon depending on its addition amount, and thus an active layer having a narrower band gap than polysilicon can be formed. By utilizing such an active layer in a TFT, a higher mobility (field effect mobility) than a TFT using an active layer of polysilicon can be realized.